1. Field of Invention
The invention relates in general to electronics and more specifically to an edge emitter display device, having particular reference to data display devices for use as a screen or display, as well as use in vacuum-tube microelectronics as super-high speed heat-and-radiation resistant devices.
2. Description of Prior Art
Known in the present state of the art is a cathode-luminescent display (cf. L'Onde Electrique, Novembre-Decembre 1991, Vol. 71, No. 6, pp. 36-42), comprising an array source of electrons and a screen situated above the surface of the source of electrons and electrically insulated from it.
The source of electrons is in fact a substrate, on which ribbon-type cathodes (arranged in columns) and gates (arranged in rows) are provided. The columns and rows are separated from one another by a dielectric layer and intersect one another. Holes are provided at the places of intersection of the ribbon-type gates (or rows) and the dielectric layer, and the holes are adapted to accept needle-type emitters whose bases are situated either directly on the ribbon-type cathode (or column) or on the layer of a load resistor applied to the ribbon-type cathodes. The tips of the needle emitters are at the level of the edges of the holes in the ribbon-type gates (or rows).
Since a display (monitor) can be either monochrome or color. A monochrome display is essentially a transparent plate on which a transparent electrically conducting coating is deposited; i.e., the first coating appearing as parallel electrodes performing the function of cathode buses (columns), and the second coating appearing as parallel electrodes performing the function of grid buses (rows), and a phosphor layer. A color display on a transparent electrically conducting layer has green, red, and blue-emitting areas of the phosphor layer, which are brought in coincidence with the areas established by the places of crossover of the ribbon-type cathodes and gates. Both the display and the source of electrons are enclosed in common air-evacuated casing.
A 400 volt constant positive voltage is applied to the display with respect to the ribbon-type cathodes, while a 50 to 80 volt constant positive voltage is applied to the ribbon-type gates with respect to the ribbon-type cathodes. In a single element or pixel cell of such an arrangement, the operation proceeds in the following manner.
Due to a short spacing between the edge of a hole in the ribbon-type gate and the tip of a needle-type emitter (i.e., of the order of 0.4-0.5 .mu.m), a high-intensity (in excess of 10.sup.7 volts per centimeter or V/cm) electric field is established at the emitter tip, and field emission of electrons from the emitter tip begins. The emitted electrons come under the effect of the accelerating electric field of the display and, while flying towards the display, the electrons bombard the phosphor, thus causing it to luminesce.
Each element (pixel) located at the crossover of the ribbon-type gate and the ribbon-type cathode provides for glow of a dot on the display. Thus, a monochrome or color picture can be established on the display by consecutively activating the respective ribbon-type gates with respect to the respective ribbon-type cathodes with a definite switch-over time.
This type of cathodoluminescent display is characterized by high voltages (that is, 400-500V) applied to the display, which results in higher power consumption which affects the operating stability and dependability of the display. During operation under the bombarding effect of the ions of the residual gases, emitter tips change geometry and undergo an increased radius of curvature which results in the lower operating stability. Ionization activity of any residual gas may occur due to a high voltage (400-500V) applied to the display and an adequately large spacing (200 .mu.m) between the tips of the emitters and the display surface. Such an increase in the radius of curvature of the emitter tips decreases the intensity of the electric field at the tips, and the field emission current is reduced, causing a resultant lower phosphor surface brightness. Such displays have but a short service life, usually not exceeding 9000 hours. Due to an increased risk of electrical breakdown between the display and the source of electrons at high anode voltages, these types of displays have had lower dependability.
Moreover, production techniques for such displays are complicated and expensive due to a sophisticated process of forming submicron-size emitting cells. These displays thus are expensive, which discourages production of cathodoluminescent displays measuring 200.times.200 mm and over.
Known in the art is another device, comprising a wedge-shaped array of field emitters and an anode positioned above the array surface (cf Wedge-shaped field emitter array for flat display, Kaneko A., Kanno T., Tomi K., Kitagawa M., and Hiraqi T. T. T. IEEE Trans Electron Devices, 1991, V. 38, No. 10, 2395-2397).
The field-emitter array in such a device is in fact a dielectric substrate, provided with parallel rows of ribbon-type aluminum cathodes and parallel rows of ribbon-type chromium gates. The rows of cathodes and of anodes intersect one another and are separated by a dielectric layer. Chromium-film emitters are provided at the places of intersection of the rows, being applied to an aluminum layer so as to form a bilateral saw-tooth pattern.
A gate is provided on the dielectric layer, having openings following the outline of the pattern of the emitters along the entire perimeter thereof with a gap of 1 .mu.m. The plane of the gate is located about 250 nm over the plane of the film emitters. The emitting surface is in effect the edge of the end face of a film emitter throughout the perimeter of the saw-tooth pattern.
The anode is essentially a glass transparent plate, having a transparent electrically conducting coating and a phosphor coating applied to its surface. The anode is spaced a few millimeters apart from the surface of the field-emitter array, and the device is hermetically sealed and air is evacuated therefrom.
At a typical one of the intersections of the rows of ribbon-type cathodes and ribbon-type gates, the operation is as follows. A 300 V constant positive voltage is applied to the anode with respect to the ribbon-type cathode, and a 50 to 80 V constant positive voltage is applied to the ribbon-type gate with respect to the ribbon-type cathode. Due to a short spacing between the edge of emitter end face and the edge of the gate hole (that is, about 1 .mu.m), a high-intensity electric field is established at the edge of the emitter end face. Field emission of electrons from the edge of the emitter is thus established. The emitted electrons come under the effect of the accelerating electric field of the anode flying towards the anode and bombarding the phosphor to cause it to luminesce. A picture can be created on the display by consecutively turning on the respective ribbon-type gates with the respective ribbon-type cathodes with a definite switch-over time.
This device features high anode voltage (+300V) and a low working pressure of residual gases. An adequately high anode voltage must be applied in order that the majority of the emitted electrons are in the anode circuit rather than in the gate circuit, and also to cause an effective phosphor luminescence, since it is seen against a light background, that is, from an anode surface devoid of phosphor.
A low pressure of the residual gases is necessary to reduce the danger of ionization of the residual gas in the space confined between the anode and the field-emitter array. Gas ionization is very much likely due to the spacing (a few millimeters) between the anode and the array. However, a low residual gas pressure is difficult to maintain in the devices during prolonged operation, due to gas entry from the surrounding atmosphere and gas coming from the structural components inside the hermetically sealed casing of the device.
Due to increased pressure in the interior of the device as time passes, high anode voltage, and large spacing between the anode and the array of the field-emission cathodes, the molecules of residual gas are ionized in the anode-to-array space. The ions so produced bombard the emitting edge of the emitter end face, thus increasing the radius of curvature of the edge. As a result, the intensity of the electric field at the edge is decreased and the magnitude of field-emission current is reduced. Furthermore, the phosphor luminance at any set voltage level is reduced, and the device thus features a low working stability over time in use. In addition, the device in question fails to provide a high-resolution (15-20 lines/mm) picture, due to a defocusing of electron beams, and also produces a harmful radiation effect due to a relatively high anode voltage.
Known in the art presently is a vacuum diode (U.S. Pat. No. 3,789,471) which comprises a substrate carrying an electrically conducting layer, and a dielectric layer carried by the electrically conducting layer and provided with a window with a cone-shaped cathode located in the window. The cathode has its base electrically contacting the conducting layer, while the tip of the emitter is at the level of another conducting layer located on the dielectric layer. The second conducting layer has a window as well, which is in register with the window of the dielectric layer. An anode is located on the conducting layer so as to hermetically seal the evacuated space established by the windows in the dielectric layer and the second conducting layer. A positive voltage is applied to the anode with respect to the cathode, and due to a short spacing between the anode and the cathode tip produces, a high-intensity electric field at the cathode tip. As a result, a field emission of electrons starts from the cathode towards the anode, and an electric current results in its circuit. Such a device can find application as a heat-and-radiation-resistant diode. The device is, however, disadvantageous in having a low time-dependent working stability, which is accounted for by the bombarding effect produced by the ions of residual gases, with the resultant increased radius of curvature of the cathode. The electric field intensity at the cathode tip thus diminishes and hence the field-emission current in the anode circuit decreases.
The above processes proceed most efficiently at a small radius of curvature of the cathode tip, while the construction of the device prevents an efficient degassing of the evaluated space by heating because the space is confined. Moreover, the materials of the vacuum diode differ in their coefficients of linear expansion, and the choice of such materials is limited by production techniques, which are very complicated and are in turn responsible for a high cost of the device.
Known in the art also is a field-emission diode (cf. Fabrication of Lateral Triode with Comb-Shaped Field-Emitter Arrays, by Junji Itoh, Kazunari Vishiki, and Kazuhiki Tsuburaya, Proceedings of the International Conference on Vacuum Microelectronics, 1993, Newport USA, pp. 99-100).
The device comprises a dielectric substrate, a film cathode (emitter), a gate, and a film anode. The gate (that is, a layer of an electrically conducting material) is located in a recess provided in the substrate between the anode and the cathode. A positive voltage (with respect to the cathode) is applied to the anode, and a positive voltage (with respect to the cathode) is applied to the gate, creating a high-intensity electric field at the edge of the cathode to establish field emission of electrons towards the end face of the anode, whereby an electric current arises in the anode circuit.
One of the disadvantages inherent in this device resides in a low operating dependability and stability due to a necessity for application of a rather high anode voltage (i.e., about 150V). This in turn adds to the danger of ionization of the residual gas molecules, while the resultant ions bombard the cathode edge, thereby changing the edge geometry and hence increasing the spacing between the anode and the edge of the cathode. As a result, the electric field intensity at the cathode edge decreases, as well as the field emission current. The risk of ionization of the residual gas molecules is also rather high in this device, due to a large distance between the emitter edge and the anode end face. Bringing the anode end face nearer to the cathode edge is a very difficult task, because the gate is interposed between the anode and cathode. Hence, an adequately high vacuum is needed for operation of the device. Because electrons are bombarding only the anode end face the device is of low dependability and it might become considerably heated and destroyed, due to high densities of the electron flow. In addition, since the electron flow does not spread over the entire surface, the device features limited functional capabilities; that is, its field of application is much restricted. Since the device requires rather high gate voltages (up to 110V) and anode voltages (up to 150V), the device consumes much power, and is disadvantageous in this respect. Also, the high voltages applied cause an increased danger of electric breakdown between the electrodes, e.g., between the cathode edge and the gate. This type of device is of low operating dependability and stability, especially under conditions of industrial vacuum, is uneconomic as to power consumption, and has but a restricted field of application.